Objective: To test the hypothesis that bone mineral content (BMC) and
density (BMD) are lost during lactation and regained within 6 months a
fter weaning. Methods: Two cohorts of women, defined by time postpartu
m were enrolled into the study; each cohort was followed for 6 months.
Women in the lactation cohort (65 lactating women and 48 nonlactating
postpartum controls) were enrolled at 2 weeks postpartum. Women in th
e weaning cohort (40 lactating and 43 nonlactating postpartum controls
) were enrolled at 4-6 months postpartum. Lactating women enrolled in
the weaning cohort had been fully breast-feeding at enrollment and wea
ned within 2 months of enrollment Bone mineral. content of the total b
ody and BMD of the lumbar spine and distal radius were measured by dua
l-energy x-ray absorptiometry. Results: Lactating women lost significa
ntly more bone in the total body (-2:8 versus -1.7%) and lumbar spine
(-3.9 versus 1.5%) than did nonlactating women during the first 6 mont
hs postpartum. There was no effect of lactation on bone changes at the
distal radius. After weaning, lactating women gained significantly mo
re bone in the lumbar spine than did nonlactating women (5.5 Versus 1.
8%). Earlier resumption of menses was associated with a smaller loss o
f bone during lactation and a greater increase of bone after weaning.
Conclusion: Women lose bone during lactation but gain bone after weani
ng. Thus, lactation may not result in net bone loss.